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A175355
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Noncomposite reverse concatenations of divisors of n, sorted by n.
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3
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1, 31, 421, 71, 131, 191, 2551, 311, 391331, 431, 48241612864321, 911371, 971, 1031, 1091, 1173913931, 1511, 1571, 1811, 1931, 2111, 2411, 24412261421, 24719131, 2711, 289171, 29214673421, 3014371, 30910331, 32565251351, 3371, 3491, 3671
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history;
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internal format)
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OFFSET
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1,2
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LINKS
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Robert Israel, Table of n, a(n) for n = 1..10000
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FORMULA
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a(n) = A176558(A089374(n)).
a(n) = A069582(n-1), n>1. [R. J. Mathar, May 03 2010]
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EXAMPLE
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For n = 9; a(9) = 391331 because A089374(9) = 39 and divisors of 39 are 1, 3, 13, 39; reverse concatenation of divisors A176558(21) = 391331 is noncomposite.
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MAPLE
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rcd:= proc(n) local D, T, i;
D:= sort(convert(numtheory:-divisors(n), list));
T:= D[1];
for i from 2 to nops(D) do
T:= T + 10^(1+ilog10(T))*D[i]
od;
T
end proc:
select(t -> t=1 or isprime(t), map(rcd, [$1..1000])); # Robert Israel, Aug 12 2020
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CROSSREFS
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Subsequence of A176558(n). Supersequence of A055781.
Sequence in context: A268137 A089375 A069582 * A068658 A068672 A274202
Adjacent sequences: A175352 A175353 A175354 * A175356 A175357 A175358
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KEYWORD
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nonn,base
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AUTHOR
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Jaroslav Krizek, Apr 20 2010
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EXTENSIONS
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Edited by Charles R Greathouse IV, Apr 30 2010
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STATUS
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approved
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